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The details depend on which expansions you're playing with, as some have starting pieces that are mutually exclusive or have slightly different rules depending on the combination. For example, the following cover all expansions through the catapult:

Collect tiles:

Collect the following special tiles, in addition to those for the base game and any other expansions being played:

Any combination of river expansion tiles (from "The River", "The River II", or the "Mini Expansion") that includes exactly one spring, exactly one lake/end tile and, optionally a single branch and a second lake/end tile.

The count piece and all 12 Carcassonne City tiles.

Prepare tiles:

Discard the starting tile; place the assembled City in the middle of the table, with the count in the castle district of the City; place the spring next to the City with the river leading away from the City, set one lake/end tile (with a volcano, if in use) and the branch tile aside, face up, and shuffle the remaining river expansion tiles and place them face down; shuffle all other tiles in use and place them in a concealed supply (in a bag, tower dispenser, or face down piles or stacks).

Drawing tiles:

Draw tiles, beginning with the branch tile, then face down river expansion tiles, then the face up lake/end tile, then from tiles in the supply.

The simplest solution would be to let the first player select a start tile at random from the pile. You can decide to ignore the other start tiles or let the players select them as alternative to a normal tile.

After the research, I think I have the best way to combine de expansions, I share it with you.

Now my expansions are: base game, river, count king and rober (with river II), traders and builders, inns a cathedrals, bridges castles and bazaars, QG#11 & phantom. This is how I'm playing the beginning of the game.

The game starts with the river fork, the river that ends with the volcano is the last for that phase, mix the spring from QG#11, 10 from the river, 10 from the river II and the original starting tile in the bag.

After all the river pieces are played, the player that place the river that end with the volcano, plays carcassonne too, choosing the location where he wants for the city, the following player starts with the rest of the pieces.

The branch is not played by a player it starts in play connected following the same line as the spring with the branch being too the left or right. Then players draw tiles from the river expansion. Furthermore, you use only two lake tiles and set one of those aside doesn't have to be the volcano though it is recommended if you are playing with it. The other lake ets shuffled with the river tiles. The river is allowed to encircle the city but it is discouraged because if the tiles don't come out in the right order there is the potential it can never be completed and you risk tiles being removed from the game.